This thesis describes the work which has been undertaken by the author in the Department of Mining and Metallurgical Engineering of the University of Queensland during the past 3 years. The major aims of this work were to determine the mechanics of the ablation process in different materials and to determine the important design parameters in the efficient operation of a jet-piercing unit. The methods available for the production of high energy flames for the penetration of rocks are studied and suggested improvements to these systems are enumerated. An analysis of the jet-piercing process has been made on the basis of the theories of rocket propulsion. Different fuel types are examined and reaction temperatures for a wide range of operating conditions have been calculated. The effects of applying extreme heat fluxes to minerals and mineral assemblages are considered on the basis of results from research in the field of ceramics. This has led to the formulation of probable ablation mechanisms in the piercing of different materials. Heat transfer from the flame to the rock and the effect of this parameter on the operating costs of the jet-piercing unit has been studied. Evidence of a heat shielding effect by ablated material has been discovered and suggestions to improve the efficiency and reduce the operating cost of a jet-piercing unit have been made. These suggestions are based on the results obtained in these investigations.